1. How come that many articles point out that movement
is a major disturbance for oximetry, yet its use for sports is
catching on? Has motion compensation technology improved that much? Or does this refer to pausing for the measurements? How often do athletes stop for the oximetry to be useful while keeeping their running pace going?

2. I want to measure SpO2 of someone in a treadmill, not real time, but analyizing the bulk of data after the session. I know about lock-in detection, where the modulated signal would be given by the heart itself, but for that technique to work I need to know the exact location of the heartbeats, so in the first pass I would extract the discrete heartbeats, and in the second pass the SpO2. I know how to do the second pass, but I don't know which technique is used for the first pass. If the heart frequency were strictly periodic, autocorrelation would work, but that's not the case here.

3. I also thought about using a green LED for heartbeat, and red and IR for SpO2. Would this work?

4. On which time scale does SpO2 rise? Say, 90% to 100% in 10 seconds?